Devices for determining the metabolic rate of a patient are generally known in the art. Known devices measure the patient's oxygen uptake by one means or another to provide a measure of metabolic rate. For example, U.S. Pat. No. 4,572,208, discloses a method and apparatus that measures oxygen uptake as an indicator of metabolic rate. Oxygen uptake can be measured: on a breath-by-breath basis, as taught by U.S. Pat. No. 4,368,740; by introducing a known amount of an inert gas (i.e., helium) into the patient's airways, as taught by U.S. Pat. No. 4,221,224; or by using an airtight system for patients under artificial respiration, as taught by U.S. Pat. No. 4,753,245.
These and other techniques require a system (either closed or open) for measuring the patient's oxygen consumption. In a closed system, a container of gas is required through which the patient inhales and exhales. Closed systems are acceptable for measurements taken under steady state conditions only. In an open system, the patient's amount of inhaled and exhaled gas are carefully monitored. In some systems, oxygen uptake is measured on a breath-by-breath basis. These systems require sophisticated sensors and gas flow meters to provide continuous output data during short transition periods.
In addition to the drawbacks noted above, a problem with using oxygen uptake as an indicator of metabolic rate is that the oxygen uptake of the human body reflects the rate of metabolism under steady state conditions only. Therefore, during transition periods in exercise and recovery, the patient's oxygen uptake cannot be used as a reliable indicator of metabolic rate.
The present invention is based upon the inventor's finding that a patient's metabolic rate ratio (MRR) can be reliably determined under both steady state and transient conditions without measuring oxygen uptake by employing data indicative of the patient's carbon dioxide and oxygen pressures of arterial blood and the patient's cardiac output. MRR is the ratio of the metabolic rate to basal rate of metabolism. In the practice of the invention, cardiac output data may be obtained by measuring heart rate and stroke volume. Various techniques have been described in the prior art for measuring heart rate. U.S. Pat. Nos. 4,034,745, 4,181,134, and 4,239,048 are representative. However, the prior art does not employ heart rate data to determine MRR, nor does the prior art provide data indicative of heart rate as well as MRR at different levels of activity.
It is, therefore, desirable to provide a method and apparatus for determining a patient's MRR at all levels of activity without complex or cumbersome equipment and without invasive physiological measurements.